The easiest way to make Scroogle your default engine is
typing
about:config
into the URL bar, and write
search
into the search box.
Almost on top should appear the line
kmeleon.general.searchEngine etc.
Click on this line and then right click it.
a dropdown menu appears, click
Modify
Replace the line in the dialog box with

and click OK.
Go then to the line below called
kmeleon.general.searchEngineName etc.
Click on this line and then right click it.
a dropdown menu appears, click
Modify
Replace Google in the dialog box with

Scroogle

and click OK.

Scroogle should now be your default search engine
for Ctrl + g , or
if you enter the keyword only into the URL bar and
press Shift + Enter, Scroogle should search for it.
Possibly Ctrl + g could have been replaced by
Shift +Enter in some builds too, as I believe to remember
having read somewhere. Check it out.

You could also make a bookmark for Scroogle Search :
Edit a given bookmark and change the URL string to
http://www.scroogle.org/cgi-bin/nbbw.cgi?Gw=%s
(Copy and paste the link, which is not just scroogle.org)
Clicking on the bookmark would also open a search prompt,
If somebody should prefer this solution.

P.S.
Yogis solution will also work, but as Scroogle is not in
the default list of search engines, the search string would first have
to be added in Edit/Preferences/Finding Websites/Search Engines,
and later be selected.